A usual multi-tubular reactor has a structure which is equipped with, as arranged within the shell of the reactor, a plurality of reaction tubes having a catalyst packed therein and a plurality of baffle plates having openings so as to distribute a heating medium introduced into the shell, entirely in the shell. It was common that the temperature of the heating medium flowing in the shell was measured, and on the basis of the result of the measurement, the operation control of the multi-tubular reactor was carried out while the temperature of the heating medium in the shell was uniformly controlled.
Most of reaction tubes disposed in the shell are connected with the baffle plates. However, some of reaction tubes passing through openings formed in the baffle plates are not connected with baffle plates. Catalyst layers in the reaction tubes, which are not in contact with the baffle plates, tend to have local heat accumulation spots (hot spots) formed due to the reaction heat. If such a hot spot was formed, the portion of the catalyst tended to be deteriorated due to an excessive heat generation, and the lifetime tended to decrease.
Further, in order to obtain proper performance in the lifetime of the catalyst by preventing formation of the hot spot, it was required to decrease the concentration of raw material gas introduced into the reaction tubes, or to limit the supply amount, whereby there was a case that (meth)acrylic acid or the like could not be produced constantly in high yield for a long period of time.